A Streptococcus sanguis and S. mutans were grown in the continuos mode both singly and in coculture. The results showed that S. sanguis had a higher affinity for glucose used as a limiting energy source than S. mutans, however, S. mutans appeared to produce an inhibitor to S. sanguis. Amino acid analysis of the culture medium following growth of these two organisms has helped to explain a discrepancy between the constant molar growth yield observed and the different fermentation products formed at high and low dilution rates. The growth parameters of some S. mutans isolates, with altered caries potential and altered growth on sucrose, have been calculated from continuos cultures. Polysaccharide fractions have been isolated from the parent and variant of an S. mutans strain altered in its colonial morphology on sucrose. The structure and composition of these polymers is now under investigation. Lactobacillus casei and S. sanguis have been grown in complex media on limiting glucose without pH control and have lowered the pH of the medium while maintaining high cell densities.